Since Sony Corporation in Japan develops lithium ion batteries in 1990, research and development of a positive material have attracted people's attention. The positive material used by the commercial lithium ion battery at present mainly is a lithium-inserted transition metal oxide, which comprises lithium cobalt oxide LiCoO2 and lithium nickelate LiNiO2 with a layer shape structure, lithium manganate LiMn2O4 with a spinel structure, and the like, wherein the most wide lithium cobalt oxide LiCoO2 material has excellent electrochemical property, but the lithium cobalt oxide LiCoO2 is short in resource, high in price, poor in safety and the like, so that large-scale application is limited especially in the field of a battery of an electric vehicle. The lithium nickelate LiNiO2 is difficult to be synthesized, the safety of which is poor, so that the application and development are limited. The lithium manganate LiMn2O4 is low in synthetic cost, good in safety, but low in capacity, and poor in high-temperature cycle stability, so that the application is limited.
Olivine-type lithium iron phosphate has become a research hotspot of the positive material for lithium ion batteries due to its excellent safety performance. Goodenough and the like researched a series of transition metal polyanionic compounds M2(XO4)3(M=TIFeNbVX=SPAsMoW) in 1997, and found out that the olivine-type lithium iron phosphate has good lithium ion intercalation capacity and deintercalation capacity. The theoretical specific capacity can be up to 170 mAh/g; the olivine-type lithium iron phosphate has more stable cycle stability than the lithium cobalt oxide; the cyclic service life can be up to over 2,000 times; a discharge platform is stable, and about at 3.4V. In addition, the lithium iron phosphate is abundant in resource, and good in environmental compatibility, and has a broad application prospect in the industry of the lithium battery, but the lithium iron phosphate has low electronic conductivity, which just is 10−9 S/cm. Pure lithium iron phosphate just can discharge 40-60% of theoretical capacity in general in a manner of adding a conductive agent when an electrode is prepared, and the first charge-discharge efficiency and the cyclic capacity retention rate are low, so that the pure lithium iron phosphate is poor in practicability when being directly applied to the positive composite material for lithium ion batteries. Modification of the lithium iron phosphate material is a sole way to make the lithium iron phosphate practical. Doping and coating are two main modification methods.
The Canada Phostech Company applies for a patented technology for coating the positive material by carbon in Canada in 1999, the patent number is CA2270771; the corresponding general formula of the positive material is AaMmZzOoNnFf, wherein A is alkali metal; M is at least one transition metal or at least one non-transition metal; Z is at least one non-metal such as O (oxygen), N (nitrogen) and F (fluorine). The positive is characterized in that carbon is deposited at the surface of AaMmZzOoNnFf, and carbon is obtained by pyrolysis of organic substances. The patented technology displays that the electrochemical property of the lithium iron phosphate after being coated by the carbon is significantly improved in comparison with that of the uncoated lithium iron phosphate. The electrical conductivity between lithium iron phosphate particles can be improved by carbon coating, also the particle size of the lithium iron phosphate also can be reduced, so that the macro electrochemical property of lithium iron phosphate is improved. But the tap density is obviously reduced due to addition of coated amorphous carbon. The tap density of the commercial lithium iron phosphate at present is smaller than 1.1 gcm−3 in general, and smaller than 1 gcm−3 in most of the time. Thus, improvement of the property of the lithium iron phosphate and improvement of the tap density cannot be organically unified.
Coating of an oxide is a common method in the field of the electrode materials for lithium ion batteries, and mainly plays the roles of improving the material stability, avoiding direct contact with electrolyte, and improving the electrochemical property of the material, for example, in US20050130042A1, oxides such as oxides of Al, Mg, Zn, Sn, Si and B coated at the surface of the LiCoO2, LiNixCo1-xO2, LiNi1/3Co1/3Mn1/3O2 and LiMn2O4.
The positive material in the US2007/0207385A1 comprises a main component A3xM12y(PO4)3, the second part of components containing at least one of SiC, BN or M22aOb; the second part is coated on the surface of the A3xM12y(PO4)3; wherein A is at least one element of IA, IIA and IIIA; M1 and M2 are at least one element of IIA, IIIA, IVA and VA. The preparation method proposed by the embodiment of the patent comprises the following steps: firstly, preparing a solution containing an A ion, an M1 ion and PO43−, or firstly preparing the A3xM12y(PO4)3; adding a solution containing the M2 ion to adjust the PH value, so as to form an M2 hydroxide sediment; and converting into an M2 oxide, so as to obtain a composite product by thermal treatment.
The patented technology mainly comprises: the oxides, SiC and BN are coated on the surface of the positive material, wherein SiC is a semiconductor; the ion is low in conductive ability, and does not have electrochemical activity. The synthetic SiC is also high in temperature (greater than 1500° C.) in general, and it is difficult to be independently coated. The SiO2 and BN belong to an insulator. Although the SiO2 has certain ion conductive ability, the effect on modification of the properties of the lithium iron phosphate is very limited by independently using SiC, SiO2, BN or any mixture thereof.